Use of surface modified porous membranes for fluid distillation

ABSTRACT

In some embodiments, the present disclosure pertains to systems and methods for distilling a fluid by exposing the fluid to a porous membrane that includes a surface capable of generating heat. In some embodiments, the heat generated at the surface propagates the distilling of the fluid by converting the fluid to a vapor that flows through the porous membrane and condenses to a distillate. In some embodiments, the surface capable of generating heat is associated with a photo-thermal composition that generates the heat at the surface by converting light energy from a light source to thermal energy. In some embodiments, the photo-thermal composition includes, without limitation, noble metals, semiconducting materials, dielectric materials, carbon-based materials, composite materials, nanocomposite materials, nanoparticles, hydrophilic materials, polymers, fibers, meshes, fiber meshes, hydrogels, hydrogel meshes, nanomaterials, and combinations thereof. Further embodiments pertain to methods of making the porous membranes of the present disclosure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/059,322, filed on Oct. 3, 2014. The entirety of theaforementioned application is incorporated herein by reference.

BACKGROUND

Current methods of distilling fluids have numerous limitations,including thermal inefficiencies, the need for heating the entire fluid,and excessive temperature polarization (in cases of membranedistillation). As such, a need exists for more effective methods andsystems for distilling fluids.

SUMMARY

In some embodiments, the present disclosure pertains to methods ofdistilling a fluid. In some embodiments, the method includes a step ofexposing the fluid to a porous membrane. In some embodiments, the porousmembrane includes a surface capable of generating heat. In someembodiments, the heat generated at the surface propagates the distillingof the fluid by converting the fluid to a vapor that flows through theporous membrane and condenses to a distillate. In some embodiments, thedistilling methods of the present disclosure also include a step ofcollecting the distillate.

In some embodiments, the fluid includes, without limitation, water,alcohols, organic solvents, volatile solvents, water-alcohol mixtures,and combinations thereof. In some embodiments, the distilling occurs bya membrane distillation method. In some embodiments, the distillingresults in fluid desalination or purification.

In some embodiments, the porous membrane includes, without limitation,polypropylene, polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polycarbonates, cellulose, and combinations thereof. Insome embodiments, the porous membrane includes a microporous membranewith pore sizes that range from about 0.2 μm to about 5.0 μm indiameter.

In some embodiments, the surface capable of generating heat spans anentire outer surface of the porous membrane. In some embodiments, thetemperature of the surface remains constant during distilling.

In some embodiments, the surface is capable of generating heat whenexposed to a light source. In some embodiments, the surface capable ofgenerating heat is associated with a photo-thermal composition thatgenerates the heat at the surface. In some embodiments, thephoto-thermal composition generates the heat at the surface byconverting light energy from a light source to thermal energy.

In some embodiments, the photo-thermal composition includes, withoutlimitation, noble metals, semiconducting materials, dielectricmaterials, carbon-based materials (e.g., carbon black, graphite,graphene, graphene oxide, reduced graphene oxide), composite materials,nanocomposite materials, nanoparticles (e.g., SiO₂/Au nanoshells),hydrophilic materials, polymers, fibers, meshes, fiber meshes,hydrogels, hydrogel meshes, nanomaterials, and combinations thereof.

In some embodiments, the photo-thermal composition includesnanoparticles. In some embodiments, the nanoparticles include, withoutlimitation, noble metal nanoparticles, metal oxide nanoparticles,semiconductor nanoparticles, gold nanoparticles, nanorods, nanoshells,SiO₂/Au nanoshells, carbon black nanoparticles, graphene nanoparticles,graphene oxide nanoparticles, reduced graphene oxide nanoparticles, andcombinations thereof.

In some embodiments, the photo-thermal composition is only associatedwith the surface of the porous membrane. In some embodiments, thephoto-thermal composition is directly associated with the surface of theporous membrane. In some embodiments, the photo-thermal composition isembedded in a polymer layer coated on the surface of the porousmembrane. In some embodiments, the photo-thermal composition is coatedon a polymer layer (e.g., polymer mesh) that is on the surface of theporous membrane.

In some embodiments, the surface is associated with the photo-thermalcomposition through at least one of covalent bonds, non-covalent bonds,physisorption, hydrogen bonds, van der Waals interactions, Londonforces, dipole-dipole interactions, and combinations thereof. In someembodiments, the photo-thermal composition is cross-linked to thesurface. In some embodiments, the surface is coated with thephoto-thermal composition. In some embodiments, the photo-thermalcomposition is cross-linked within the coating.

In some embodiments, the methods of the present disclosure also includea step of exposing the surface of the porous membrane to a light source.In some embodiments, the light source facilitates heat generation by thesurface. In some embodiments, the light source includes sunlight.

In some embodiments, the methods of the present disclosure occur withoutheating of the bulk fluid (e.g., without heating of the entire fluid orgeneralized heating). In some embodiments, the methods of the presentdisclosure occur by only heating the fluid near the surface of theporous membrane. In some embodiments, the method occurs without the useof electric energy.

In some embodiments, the present disclosure pertains to systems fordistilling a fluid. In some embodiments, the systems of the presentdisclosure include the porous membranes of the present disclosure.Further embodiments of the present disclosure pertain to methods ofmaking the porous membranes.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a method (FIG. 1A) and a system (FIG. 1B) fordistilling fluids using membrane distillation.

FIG. 2 shows the temperature profile and heat fluxes in membranedistillation (MD). FIG. 2A shows temperature polarization in current MDtechnology. FIG. 2B shows a porous membrane that is coated with aphoto-thermal composition with reverse temperature polarization.

FIG. 3 shows examples of physical and chemical methods for attachment ofphoto-thermal compositions to base membrane surfaces, including SiO₂/Aunanoparticles (FIG. 3A), unmodified carbon black nanoparticles (CB NPs)(FIG. 3B), functionalized CB NPs (FIG. 3C), and cross-linked grapheneoxide (GO) (FIG. 3D). The electrospinning scheme illustrated in FIG. 6can be utilized to apply the photo-thermal compositions to base membranesurfaces.

FIG. 4 shows a diagram of a direct contact membrane distillation (DCMD)experimental setup.

FIG. 5 shows a change of permeate mass over time for variouspolyvinylidene fluoride (PVDF) membranes, including a nanomaterial (NM)coated PVDF membrane (FIG. 5A), a PVDF membrane with an unmodified nylonmesh and NM coated meshes (FIG. 5B), and a base PVDF membrane with anelectro-spinning coated (polymethyl methacrylate) PMMA/CB NP polymerlayer (FIG. 5C).

FIG. 6 shows an illustration of an apparatus for electrospinning aphoto-thermal composition onto a porous membrane to form a photo-thermalnanocomposite fiber mesh.

FIG. 7 provides scanning electron microscopy (SEM) images of the surface(FIG. 7A) and cross-section (FIG. 7B) of electrospun PMMA-CBNP fibermeshes (M-ESPCs). An SEM image of the surface of an electrospunpoly(vinyl alcohol) (PVA) hydrogel fiber with CBNP (HM-ESPC) is alsoshown (FIG. 7C).

FIG. 8 shows data relating to various fiber diameters. FIG. 8A shows ahistogram of the M-ESPC fiber diameters. FIG. 8B shows a chart of fiberdiameters for various HM-ESPCs with different concentrations of CBNPs.

FIG. 9 shows liquid entry pressure (LEP) measurements of differentmembrane samples. Error bars are the maximum possible error from theprecision of the pressure gauge (2.5 psi).

FIG. 10 shows permeate flux measurements of different membrane samples.The measurements are shown with and without light irradiance.

FIG. 11 shows data indicating that HM-ESPC coating thickness increaseswith electrospinning time.

FIG. 12 shows data relating to deionized water contact anglemeasurements on membranes modified with hydrophilic electrospun fibers.FIG. 12A shows hydrophobic PVDF, electrospun mats of hydrophobic PMMAfibers, and PVDF membranes modified with CBNP. FIG. 12B shows PVDFmembranes modified with hydrophilic PVA fibers with differentelectrospinning times (5 minutes to 2 hours). Bars with blue hash markscontain 2% CBNP.

DETAILED DESCRIPTION

It is to be understood that both the foregoing general description andthe following detailed description are illustrative and explanatory, andare not restrictive of the subject matter, as claimed. In thisapplication, the use of the singular includes the plural, the word “a”or “an” means “at least one”, and the use of “or” means “and/or”, unlessspecifically stated otherwise. Furthermore, the use of the term“including”, as well as other forms, such as “includes” and “included”,is not limiting. Also, terms such as “element” or “component” encompassboth elements or components comprising one unit and elements orcomponents that comprise more than one unit unless specifically statedotherwise.

The section headings used herein are for organizational purposes and arenot to be construed as limiting the subject matter described. Alldocuments, or portions of documents, cited in this application,including, but not limited to, patents, patent applications, articles,books, and treatises, are hereby expressly incorporated herein byreference in their entirety for any purpose. In the event that one ormore of the incorporated literature and similar materials defines a termin a manner that contradicts the definition of that term in thisapplication, this application controls.

Purified fluids play essential roles in everyday life. For instance, noother resource is as universally necessary for life as is water.Moreover, the safety and availability of purified water is a grandchallenge inextricably linked to global health, energy production andeconomic development.

However, access to clean water is a fast-growing challenge due topopulation growth, increased water pollution by recalcitrant andhazardous contaminants, and climate change that threatens to exacerbatewater scarcity in many areas. As such, a need exists for technologicalinnovation to tap unconventional water sources and meet the fast-growingdemand for affordable water.

Extracting clean water from saline (e.g., seawater and brackishgroundwater) or contaminated water sources holds tremendous potential inmeeting the world's current and future water needs. For instance,desalination and water purification by evaporation or reverse osmosishas become an important water source in many parts of the world. In2004, 3.24×10⁷ m³/day of fresh water was produced by desalination plantsworldwide.

However, desalination is a highly energy intensive process. Consideredas the most energy efficient desalination technology, reverse osmosis(RO) still consumes 1.5 to 2.5 kwh/m³ to desalinate seawater. Moreover,the complex, high pressure system requires significant capitalinvestment and extensive pretreatment in order to control membranefouling.

For instance, in many locations in the developing world where sufficientand safe water supply is lacking, there is no access to electricity.Moreover, limited fund is available for purchasing equipment.Furthermore, trained operators are not available to run a complex waterpurification system.

Accordingly, off-grid, simple, low cost and high efficiency waterdesalination and purification systems are greatly needed. Solar membranedistillation is a technology that could potentially meet this need.

Membrane distillation (MD) utilizes the vapor pressure difference acrossa porous, hydrophobic membrane that separates the hot feed stream andthe cold permeate stream to drive flux of water vapor from the feedstream to the permeate stream and generate purified water uponcondensation (FIG. 2A). The water vapor flux through the porous membraneJ can be described by Equation 1:

J=k _(m) [p(T ₁)-p(T ₂)]  (1)

Here, k_(m) is the water vapor permeability of the membrane (m/s-mmHg);T₁ and T₂ (° C.) are the temperature at the membrane surface on the feedand permeate side, respectively; and p (mmHg) is the vapor pressure ofwater, which can be estimated using the Antoine Equation (Equation 2):

$\begin{matrix}{{\log_{10}p} = {8.07131 - {\frac{1730.63}{233.426 + T}( {{1{^\circ}\mspace{14mu} {C.}} \leq T \leq {100{^\circ}\mspace{14mu} {C.}}} )}}} & (2)\end{matrix}$

MD has several unique advantages over other desalination technologies.For instance, since MD operates below the boiling point of water, itsthermal energy consumption is significantly lower than other evaporationbased desalination processes. Moreover, the flow of water vapor throughthe membrane is driven by the partial pressure difference due to thetemperature gradient instead of trans-membrane pressure. Therefore,electric energy consumption and requirement for membrane mechanicalstrength is low. In addition, as the partial pressure of water vapor isnot significantly affected by salinity, MD can be applied to water withvery high salinity and without significantly affecting the energyconsumption.

Furthermore, because there is no water flow through the membrane,membrane fouling is much less in MD than in RO systems. In addition,unlike RO, MD can operate intermittently without damaging the membrane.Finally, the low operating temperature and the flexibility of operationcycles make MD an optimal candidate for solar desalination. Accordingly,the solar energy is harvested to both heat the feed water and generateelectricity for pumping.

A solar MD system uses a solar thermal collector to heat the feed watereither directly or through heat exchange with a working fluid and aphotovoltaic device to generate the electricity needed for pumping.Because the majority of the energy use in MD is thermal energy, theefficiency of the solar thermal collector and the thermal efficiency ofthe MD unit determine the overall energy efficiency of a solar MDsystem. Although the optical efficiency of various solar collectors(defined as the fraction of solar irradiation that is absorbed) can beas high as 80%, the heat loss in solar collectors reduces the totalefficiency to 20-70%, depending on the type of collectors and theoperating temperature.

In the MD unit, there are two major sources of energy loss. The firstsource is temperature polarization. Due to conductive and latent heattransfer, the temperature at the membrane surface on the feed side T₁can be significantly lower than that in the bulk feed solution T_(f)(FIG. 2A). Similarly, T₂ at the membrane surface on the permeate side ishigher than the temperature of the bulk permeate water T_(p).Temperature polarization could reduce the transmembrane temperaturegradient by up to 70%, which greatly reduces the driving force for vaportransport. A temperature polarization coefficient α_(TP) can be measuredin accordance with Equation 3:

$\begin{matrix}{\alpha_{TP} = \frac{T_{1} - T_{2}}{T_{f} - T_{p}}} & (3)\end{matrix}$

Furthermore, the heat transfer across the membrane causes diminishedtemperature difference between feed and permeate along the membranelength, which limits the production rate of an MD unit and poses a majorscale-up challenge.

Another important source of energy loss is the brine. Heat loss throughthe discharge of the brine increases with decreasing water recovery. Thesingle pass water recovery in solar MD is typically below 5% (i.e., 95%of the heat in the feed water is lost). With brine recirculation,product water recovery for MD can reach 65% to 95%, but this increasessystem complexity and pumping cost.

In addition, high water recovery may lead to scaling. Heat exchangerscan be used to recover heat from permeate and brine, but that also addssignificantly to system complexity and cost.

In sum, current MD processes have several limitations. For instance,current MD processes require high thermal energy consumption.Electricity is used to heat the feed water to a high temperature todrive the MD process. Therefore, the electric energy consumption is veryhigh. Moreover, temperature polarization (i.e., lower temperature at themembrane surface on the feed side than the bulk feed water, and highertemperature at the membrane surface on the permeate side than the bulkpermeate water) leads to reduced driving force and hence lower watervapor flux. In addition, the driving force decreases with flow channellength, thereby limiting the effective size of the membrane module.Moreover, current MD processes show a loss of residual heat in the brinedischarge.

Furthermore, current solar membrane distillation systems useconventional solar collectors to heat up the feed water directly orthrough a heat exchanger. However, since water is a poor light absorber,direct heating of feed water by sun light is very inefficient. Inaddition, installing solar collectors and heat exchangers add complexityand cost to solar systems. In addition, since current solar MD systemsheat up the bulk volume of feed water, significant thermal energy islost via conduction across the membrane and hot brine discharge.

As such, a need exists for improved MD systems that have high thermalefficiency. A need also exists for improved MD systems that only heatthe fraction of fluid recovered as permeate. In addition, a need existsfor improved MD systems that have negligible temperature polarization.The present disclosure addresses these needs.

In some embodiments, the present disclosure pertains to methods ofdistilling a fluid. In some embodiments illustrated in FIG. 1A, suchmethods include one or more of the following steps: exposing the fluidto a porous membrane with a surface capable of generating heat (step10); generation of heat at the surface of the porous membrane (step 12);converting the fluid to a vapor (step 14); flowing the vapor through theporous membrane (step 16); condensing the fluid to a distillate (step18); and collecting the distillate (step 20).

In some embodiments, the present disclosure pertains to systems fordistilling a fluid. In some embodiments illustrated in FIG. 1B, system10 can be utilized to distill fluids. System 10 can include a porousmembrane 16 with surface 18 that is capable of generating heat. In someembodiments, system 10 may also include a channel 14 for exposing fluid12 to surface 18 of porous membrane 16. In some embodiments, system 10may also include a channel 26 for flowing distillate 24, and a container30 for collecting distillate 24. System 10 may also include channel 28for flowing permeate fluid 22 through system 10. In addition, system 10may be associated with light source 20.

In operation, fluid 12 can flow through channel 14 for exposure tosurface 18 of porous membrane 16. Light source 20 can be used tofacilitate the heating of surface 18. Thereafter, heat generated atsurface 18 of porous membrane 16 propagates the distilling by convertingfluid 12 to a vapor that flows through the porous membrane 16 andcondenses to a distillate 24 that flows through channel 26 and intocontainer 30. Permeate fluid 22 can also propagate the distilling byfacilitating the condensation.

Further embodiments of the present disclosure pertain to methods ofmaking membranes for fluid distillation. As set forth in more detailherein, various methods and systems may be utilized to distill varioustypes of fluids. Moreover, various porous membranes may be utilized inthe systems and methods of the present disclosure. Furthermore, variousmethods may be utilized to make the porous membranes of the presentdisclosure.

Porous Membranes

The methods and systems of the present disclosure can utilize varioustypes of porous membranes. In some embodiments, the porous membranes ofthe present disclosure include porous membranes that serve as barriersbetween a fluid feed and permeate while allowing vapor to pass. In someembodiments, the porous membranes of the present disclosure arenon-wetting membranes. In some embodiments, the porous membranes of thepresent disclosure are hydrophobic. In some embodiments, the porousmembranes of the present disclosure have high hydrophobicity, chemicalstability, and thermal stability.

The porous membranes of the present disclosure can have variouscompositions. For instance, in some embodiments, the porous membranes ofthe present disclosure can include, without limitation, polypropylene(PP), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),polyethylene, polycarbonates, cellulose, and combinations thereof.

The porous membranes of the present disclosure can have variousporosities. For instance, in some embodiments, the pores in the porousmembranes include diameters between about 1 nm to about 5 μm. In someembodiments, the pores include macropores with diameters of at leastabout 50 nm. In some embodiments, the pores include macropores withdiameters between about 50 nm to about 3 μm. In some embodiments, thepores include macropores with diameters between about 500 nm to about 2μm. In some embodiments, the pores include mesopores with diameters ofless than about 50 nm. In some embodiments, the pores include microporeswith diameters of less than about 2 nm.

In some embodiments, the porous membranes of the present disclosureinclude a microporous membrane. In some embodiments, the porousmembranes of the present disclosure include pore sizes that range fromabout 0.2 μm to about 5.0 μm in diameter. In some embodiments, theporous membranes of the present disclosure include pore sizes that rangefrom about 0.2 μm to about 1 μm in diameter. In some embodiments, theporous membranes of the present disclosure include pore sizes that rangefrom about 1 μm to about 5 μm in diameter.

The porous membranes of the present disclosure can also have variousshapes. For instance, in some embodiments, the porous membranes of thepresent disclosure are in the form of flat sheets. In some embodiments,the porous membranes of the present disclosure have a rectangular shape.Additional porous membrane shapes can also be envisioned.

In some embodiments, the porous membranes of the present disclosureinclude flat sheets of PTFE, PVDF and PP with pore sizes ranging fromabout 0.2 μm to about 1.0 μm in diameter. In some embodiments, theporous membranes of the present disclosure include hydrophobic andhydrophilic membranes with large pore sizes (e.g., PVDF, polycarbonate,and mixed cellulose ester porous membranes with pore sizes that rangefrom about 1 μm to about 5 μm in diameter).

Porous Membrane Surfaces

The porous membrane surfaces of the present disclosure can have variousattributes. For instance, in some embodiments, the porous membranes ofthe present disclosure include a surface that is capable of generatingheat. In some embodiments, the surface is hydrophobic. In someembodiments, the heat is only generated at the surface of the porousmembrane. In some embodiments, the heat generated at the surface of theporous membrane propagates the distilling of the fluid. In someembodiments, the surface capable of generating heat spans an entireouter surface of the porous membrane (e.g., surface 18 on porousmembrane 16 in FIG. 1B). In some embodiments, the temperature of thesurface remains constant during distilling. In some embodiments, thesurface is capable of generating heat when exposed to a light source.

Photo-thermal Compositions

In some embodiments, the porous membranes of the present disclosure arealso associated with a photo-thermal composition. In some embodiments,the photo-thermal composition is capable of generating heat. Forinstance, in some embodiments, a porous membrane surface capable ofgenerating heat is associated with a photo-thermal composition. In someembodiments, the photo-thermal composition generates the heat at thesurface. In some embodiments, the photo-thermal composition generatesthe heat at the surface by converting light energy from a light sourceto thermal energy.

The porous membrane surfaces of the present disclosure may be associatedwith various photo-thermal compositions. In some embodiments, thephoto-thermal composition is hydrophilic. In some embodiments, thephoto-thermal composition includes, without limitation, noble metals,semiconducting materials, dielectric materials (e.g., Au nanoparticles),carbon-based materials, composite materials, nanocomposite materials,nanoparticles (e.g., SiO₂/Au nanoshells), hydrophilic materials,polymers, fibers, meshes, fiber meshes, hydrogels, hydrogel meshes,nanomaterials, and combinations thereof.

In some embodiments, the photo-thermal compositions of the presentdisclosure include carbon-based materials. In some embodiments, thecarbon-based materials include, without limitation, carbon black,graphite, graphene, graphene oxide, reduced graphene oxide, andcombinations thereof.

In some embodiments, the photo-thermal composition includes a polymer.In some embodiments, the polymer includes, without limitation,hydrophillic polymers, polymer fibers, electrospun polymers,functionalized polymers, and combinations thereof. In some embodiments,the photo-thermal composition includes a hydrophilic polymer, such as apoly(vinyl alcohol) (PVA). In some embodiments, the photo-thermalcomposition includes, without limitation, hydrophilic PVA fibers,electrospun PVA hydrogel fibers, hydrogel meshes, hydrogel fibers, andcombinations thereof.

In some embodiments, the photo-thermal composition includesnanoparticles. In some embodiments, the nanoparticles include, withoutlimitation, noble metal nanoparticles, metal oxide nanoparticles,semiconductor nanoparticles, gold nanoparticles, nanoshells, SiO₂/Aunanoshells, nanorods, carbon black (CB) nanoparticles, graphenenanoparticles, graphene oxide (GO) nanoparticles, reduced graphene oxidenanoparticles, and combinations thereof.

In some embodiments, the nanoparticles include one or more noble metals.In some embodiments, high nanoparticle concentrations can result inmultiple scattering by neighboring nanoparticles that increase lightabsorption. In some embodiments, the nanoparticles are in the form ofnoble metals that are surface plasmon resonant.

The porous membrane surfaces of the present disclosure may be associatedwith photo-thermal compositions in various manners. For instance, insome embodiments, the surface capable of generating heat is associatedwith the photo-thermal composition through at least one of covalentbonds, non-covalent bonds, physisorption, hydrogen bonds, van der Waalsinteractions, London forces, dipole-dipole interactions, andcombinations thereof. In some embodiments, the photo-thermal compositionis cross-linked to a surface.

In some embodiments, the surface capable of generating heat is coatedwith the photo-thermal composition. In some embodiments, thephoto-thermal composition is cross-linked within the coating.

In some embodiments, the photo-thermal composition is embedded in apolymer layer coated on the surface of the porous membrane. In someembodiments, the photo-thermal composition is coated on a polymer layer(e.g., polymer mesh) that is on the surface of the porous membrane. Insome embodiments, the polymer layer is made of transparent materials. Insome embodiments, the polymer layer includes, without limitation,polystyrenes, polyacrylonitriles, polymethyl methacrylates,polydopamine, and combinations thereof.

In some embodiments, the photo-thermal compositions of the presentdisclosure are only associated with the surface of the porous membranethat is capable of generating heat. In some embodiments, thephoto-thermal composition is directly associated with the surface of theporous membrane that is capable of generating heat. In some embodiments,the remaining portions of the porous membranes lack the photo-thermalcompositions. In some embodiments, the photo-thermal compositions do notpenetrate the pores of the porous membrane.

In some embodiments where the photo-thermal compositions includenanoparticles, the nanoparticles may form an array on a surface of aporous membrane. In some embodiments, the nanoparticles are adjacent toone another. In some embodiments, the nanoparticles are embedded in ahydrophilic material coated on the surface. In some embodiments, thehydrophilic material includes, without limitation, polymers, meshes,fibers, mats, hydrogels, and combinations thereof. In some embodiments,the photo-thermal compositions of the present disclosure are in the formof nanoparticles that are covalently bound to a porous membrane surface.

Light Sources

In some embodiments, the methods of the present disclosure also includea step of exposing the surface of a porous membrane that is capable ofgenerating heat to a light source. In some embodiment, the light sourcefacilitates heat generation by the surface.

Various light sources may be utilized to generate heat at a surface. Forinstance, in some embodiments, the light source includes, withoutlimitation, natural light (i.e., sunlight), incident light, visiblelight, ultraviolet light, near infrared light, laser, continuous wavelaser, incandescent light, fluorescent light, LED light, light derivedfrom solar radiation (e.g., light derived from solar panels), engineeredlight sources, and combinations thereof. In some embodiments, the lightsource includes sunlight.

In some embodiments, the light intensity from the light source isamplified by a light amplifier. In some embodiments, the light amplifierincludes optical lenses.

In some embodiments, light sources are exposed to a surface of a porousmembrane that includes photo-thermal compositions. In some embodiments,the photo-thermal compositions that are exposed to a light sourceinclude nanoparticles (as previously described).

In some embodiments, the nanoparticles are in the form of noble metalsthat are surface plasmon resonant. In some embodiments, thenanoparticles have semiconducting properties. Without being bound bytheory, it is envisioned that noble metal NPs have abundant mobileelectrons and therefore outstanding photo-thermal efficiency, which isfurther enhanced when the incident light wavelengths are near theirsurface plasmon resonance. Moreover, it is envisioned that, in suchnoble metal NPs, free electrons strongly absorb light across the UV tonear infrared (NIR) wavelength range. These energetic electrons undergoelectron-electron scattering, and rapidly (in picoseconds) transfer thekinetic energy to the lattice of the NP through electron-phononinteraction, as illustrated in Equation 4:

$\begin{matrix}{\tau_{r} = \frac{r_{p}^{2}}{6.75\alpha_{p}}} & (4)\end{matrix}$

When the irradiation pulse is shorter than the relaxation time τ_(r)(Eq. 4) of a noble metal NP with a radius r_(p) and thermal diffusivityα_(p), heating is confined in the noble metal NP with negligible heatloss on timescales <τ_(r), leading to rapid increase in the temperatureof the NP. At slower irradiation rate, the thermal energy is thentransferred to the surrounding fluid through phonon-phonon coupling.

When the noble metal NP is submerged in water, superheating of water upto its spinodal decomposition temperature at the nanoparticle-waterinterface has been reported. With multiple NPs, the heating effect isstrongly enhanced not only due to the accumulative effect (multiple heatsources), but also the interaction between plasmon-enhanced electricfields of neighboring plasmonic NPs (Coulomb interaction).

In some embodiments, the nanoparticles of the present disclosure canalso include SiO₂/Au nanoshells, carbon black (CB) NPs, graphene oxide(GO) NPs, and combinations thereof. In some embodiments, such NPs areadvantageous for use as photo-thermal compositions because they absorbstrongly across the whole solar spectrum.

For instance, a core-shell structure such as that in a SiO₂/Au nanoshellallows the plasmon resonance to be tuned to better match the incidentlight spectrum. Likewise, CB NPs are low-cost heat-conducting materials,and have been used in solar collectors as nanofluids for enhanced heatgeneration. Similarly, GO strongly absorbs over a wide spectrum rangingfrom UV to near infrared (NIR), and has been utilized in photo-thermaltreatment of cancer cells using NIR lasers. More importantly, watervapor exhibits unique transport behavior through stacked GO sheets.

Exposing of Porous Membranes to Fluids

Various methods may also be utilized to expose the porous membranes ofthe present disclosure to fluids. For instance, in some embodiments, theporous membranes of the present disclosure are exposed to fluids byflowing the fluids through a channel that contains a porous membrane(e.g., channel 14 in FIG. 1B). In some embodiments, the fluid becomesassociated with the surface of the porous membrane through directcontact with the surface. In some embodiments, the porous membranes ofthe present disclosure are exposed to fluids by incubating the fluidswith the porous membrane. Additional exposure methods can also beenvisioned.

Fluids

The methods and systems of the present disclosure may be utilized todistill various types of fluids. For instance, in some embodiments, thefluid includes, without limitation, water, alcohols, organic solvents,volatile solvents, water-alcohol mixtures, and combinations thereof. Insome embodiments, the fluid includes an ethanol-water mixture. In someembodiments, the fluid includes water.

Propagation of Distillation

Various methods may be utilized to propagate the distilling of a fluidby a porous membrane. For instance, in some embodiments, the heatgenerated at a surface of the porous membranes propagates the distillingby converting the fluid to a vapor that flows through the porousmembrane and condenses to a distillate. In some embodiments, the heatgenerated at the surface propagates the distilling by creating atemperature gradient across the porous membrane. Thereafter, thetemperature gradient results in the formation of a vapor pressuregradient that drives the formed vapor through the porous membrane.

Collecting

In some embodiments, the methods of the present disclosure may alsoinclude a step of collecting the distillate. Various methods may beutilized to collect a distillate. For instance, in some embodiments, thedistillate may be collected by flow through a channel (e.g., channel 26in FIG. 1B) and into a container (e.g., container 30 in FIG. 1B). Insome embodiments, the distillate can be collected by a liquid flow, anair flow, or a vacuum. In some embodiments where the distillate iscollected by an air flow or a vacuum, the vapor in the gas phase iscondensed using a heat exchanger.

Distillation Methods

The methods and systems of the present disclosure may be applied tovarious distillation methods. For instance, in some embodiments, thedistilling occurs by a membrane distillation method. In someembodiments, the membrane distillation method includes, withoutlimitation, direct-contact membrane distillation, air-gap membranedistillation, sweeping-gas membrane distillation, vacuum membranedistillation, and combinations thereof.

In some embodiments, the distilling results in fluid desalination. Insome embodiments, the distilling results in fluid purification. In someembodiments, the distilling results in solvent separation. In someembodiments, the distilling occurs without heating of bulk fluid. Insome embodiments, the distilling occurs by only heating the fluid nearthe surface of the porous membrane. In some embodiments, the distillingoccurs without the use of electric energy.

Methods of Making Membranes

In some embodiments, the present disclosure pertains to methods ofmaking the porous membranes of the present disclosure. In someembodiments, the methods of the present disclosure include a step ofassociating a surface of a porous membrane with a photothermalcomposition.

Suitable porous membranes were described previously. In someembodiments, the porous membrane includes, without limitation,polypropylene, polyvinylidene fluoride, polytetrafluoroethylene,polyethylene, polycarbonates, cellulose, and combinations thereof.

Suitable photo-thermal compositions were also described previously. Insome embodiments, the photo-thermal compositions of the presentdisclosure include, without limitation, noble metals, semiconductingmaterials, dielectric materials, carbon-based materials, compositematerials, nanocomposite materials, nanoparticles, hydrophilicmaterials, polymers, fibers, meshes, fiber meshes, hydrogels, hydrogelmeshes, nanomaterials, and combinations thereof.

Various methods may be utilized to associate a surface of a porousmembrane with photo-thermal compositions. For instance, in someembodiments, the porous membrane surfaces of the present disclosure canbe associated with photo-thermal compositions by immobilization methods.In some embodiments, the association methods can include, withoutlimitation, coating, spraying, drop-casting, dip-coating, covalentbinding (e.g., in wet chemistry), polymer blend coating, covalentcrosslinking, spinning, electrospinning, and combinations thereof. Insome embodiments, the surfaces of the porous membranes of the presentdisclosure are associated with photo-thermal compositions byelectrospinning the photo-thermal composition onto the surface of theporous membrane. In some embodiments, the photo-thermal composition isin the form of electrospun fiber meshes.

As also described previously, porous membranes can become associatedwith photo-thermal compositions in various manners. For instance, insome embodiments, the surface becomes associated with the photo-thermalcomposition through at least one of covalent bonds, non-covalent bonds,physisorption, hydrogen bonds, van der Waals interactions, Londonforces, dipole-dipole interactions, and combinations thereof.

In some embodiments, the photo-thermal composition becomes directlyassociated with the surface of the porous membrane. In some embodiments,the photo-thermal composition becomes embedded in a polymer layer coatedon the surface (e.g., polymer layers such as polystyrenes,polyacrylonitriles, polymethyl methacrylates, polydopamine, andcombinations thereof). In some embodiments, the nanoparticles becomeembedded in a hydrophilic material coated on the surface (e.g.,hydrophilic materials such as polymers, meshes, fibers, mats, hydrogels,and combinations thereof). In some embodiments, the photo-thermalcomposition becomes cross-linked to the surface. In some embodiments,the surface becomes coated with the photo-thermal composition. In someembodiments, the photo-thermal composition becomes cross-linked withinthe coating.

Applications and Advantages

The methods and systems of the present disclosure provide numerousadvantages. For instance, in some embodiments, the photo-thermalcompositions of the present disclosure can generate heat from lightsources such as sunlight and thereby create a high temperature at themembrane surface to drive the distillation process without usingelectricity or specialized equipment (e.g., heat exchangers, solarcollectors, and the like). In addition, the temperature polarization canbe reversed. Moreover, the temperature at the porous membrane surfacecan remain relatively constant throughout a flow channel. Therefore, thedistillation systems of the present disclosure can be made larger forhigher fluid distillation rates. Furthermore, since the feed can only beheated at the membrane surface, there is minimum residual heat in thebrine.

As such, in some embodiments, the systems and methods of the presentdisclosure can greatly improve the energy efficiency of current membranedistillation processes by using sunlight as the energy source, andmaximizing sunlight absorbance and photo-thermal conversion, as well asreducing heat loss through brine discharge. Moreover, the systems andmethods of the present disclosure can increase fluid recovery of asingle pass membrane distillation unit by allowing longer feed channeland hence larger membrane size.

The aforementioned improvements overcome several barriers of currentmembrane distillation technologies. For instance, the capability ofphoto-thermal compositions of the present disclosure to heat fluids(e.g., feed water) makes it possible to operate solar membranedistillation without the need for a solar collector or any heatexchanger. This is expected to greatly reduce the overall complexity andcapital cost of the systems and methods of the present disclosure,thereby making them good candidates for providing purified water atremote locations where electricity and safe water supply are notavailable.

Additional Embodiments

Reference will now be made to more specific embodiments of the presentdisclosure and experimental results that provide support for suchembodiments. However, Applicants note that the disclosure below is forillustrative purposes only and is not intended to limit the scope of theclaimed subject matter in any way.

EXAMPLE 1 Enhancement of the Energy Efficiency of Direct Solar MDProcesses

In this Example, Applicants utilize the high photo-thermal efficiencyand the localized heating of selected nanomaterials (NMs) to enhance theenergy efficiency of the direct solar membrane distillation (MD) process(i.e., no solar collector or heat exchangers). At high nanoparticle (NP)concentrations, multiple scattering by neighboring NPs concentratesphotons and increases light absorption (FIG. 2B).

High concentrations of photo-thermal NPs immobilized on MD membranesurfaces (feed side) can serve as efficient localized heat sources whenirradiated by sunlight. The localized heating creates higher temperatureon the membrane surface (T₁) than the feed water (T_(f)), and henceincreases water vapor flux without significantly heating the bulk feedwater. This process is expected to have higher energy efficiency thanexisting solar thermal MD processes, which create the needed temperaturedifference by heating the whole volume of the feed water, although onlya fraction of it is converted to the clean, permeate water. The highlocal temperature at the membrane surface reverses temperaturepolarization (FIG. 2B). Therefore, the high flow rate required toenhance heat transfer in the feed channel in order to minimizetemperature polarization is no longer necessary. Furthermore, with themodified membrane, heat is generated at the whole membrane surface,which effectively alleviates the diminished temperature difference alongthe membrane length, and enables possibility for larger scale MDmodules.

EXAMPLE 2 Preparation and Characterization of MD Membranes withPhoto-thermal Compositions

Photo-thermal nanocomposite membranes can be created by 1), directlycoating the photo-thermal NMs on the surface of MD membranes, or 2), byincorporating the NMs in a polymer layer which sits on top of the MDmembrane. The advantage of using an additional polymer layer is toachieve higher coating density of the photo-thermal NMs. Depending onthe type of NM used, different coating/incorporation methods are neededto ensure proper attachment and concentration of the NMs whilemaintaining other important properties of the membrane, such as highsurface hydrophobicity, low thermal conductivity, and high porosity.Because SiO₂/Au nanoshells and carbon black (CB) NPs are smaller thanthe membrane pore sizes, and because both Au/SiO₂ nanoshells andoxidized CB NPs are very hydrophilic, a consideration in choosing acoating method is prevention of NM penetration into membrane pores.Penetration of the NMs into the membrane pores may not only lead to adecrease in porosity and hydrophobicity of the pores, but also theheating of the whole membrane matrix, which in turn may cause a decreasein a trans-membrane temperature difference.

Example 2.1 Coating of Photo-thermal Nanomaterials on PolydopamineModified MD Membranes

A polydopamine coating can be used to facilitate adhesion of certainphoto-thermal NMs on the membrane surface (FIG. 3A). Polydopamine caneffectively coat a wide range of substrate surfaces, and the catecholgroups in polydopamine has high binding affinity to metals. This allowsattaching photo-thermal NMs with metal surface on various base membranesurfaces without the need for functionalizing the base membranes. Thisfeature is particularly attractive for PTFE or PVDF membranes, whosesurfaces are highly chemically inert. The candidate NMs for this coatingapproach can include, without limitation, noble metal NPs, such as goldNPs; and core-shell structure NMs, such as SiO₂/Au nanoshells, SiO₂/Aunanorods, and the like.

The base membrane samples can be mounted on a glass slide with theactive layer facing up to protect the back side of the membrane fromcontacting the coating solution. Coating can be achieved by a simplesoaking method or dip coating method. The dry membrane sample can firstbe exposed to a dopamine solution buffered at pH˜8.5. Thepolydopamine-coated membrane can be dried and then exposed to aphoto-thermal metal NM suspension. The immersion time, the dippingspeed, number of dip cycles as well as dopamine and nanoshellconcentrations can be tested and optimized to achieve uniform coatingsof different nanoshell surface loadings while avoiding dopaminepenetration and nanoshell deposition in the pores.

Preliminary tests with a 0.2 μm PVDF membrane showed notable decrease inmembrane surface hydrophobicity after soaking in dopamine solution for15 minutes. However, no pore flooding was observed in the DCMDexperiments using the composite membranes prepared. This suggests that,by limiting contact time with dopamine, one can limit the polydopaminecoating on the membrane surface but not in the membrane pores.

EXAMPLE 2.2 Direct Coating of Photo-Thermal NMs on MD Membranes

Typical MD membranes are preferably hydrophobic. Thus, photo-thermal NMsthat are hydrophobic in nature can adhere well to MD membrane surfacesthrough strong van der Waals forces (FIG. 3B). Drop casting, spincoating and dip coating can be used for attaching the hydrophobicphoto-thermal NMs on the membrane. The candidate photo-thermal NMs caninclude, without limitation CB NPs or reduced GO.

Because hydrophobic NMs may not disperse in water, a suitable solvent isneeded. The solvent desirably allows good dispersion of the NMs, but nopenetration of CB NPs into membrane pores. It is also desirable for thesolvent not to damage the membrane material. Although PVDF and PTFE arehighly resistant to most solvents, PP (which is also used as the supportlayer in PTFE membranes) is more subject to attack by solvents.

In preliminary studies, chloroform was used to disperse CB NPs.Chloroform disperses CB NPs very well. A simple soaking and evaporationprotocol was able to provide good CB coating on a PTFE membrane. Nosignificant detachment of CB NPs was observed, even when physicalscrubbing was applied to the modified membrane surface. However,chloroform can wet PTFE and PVDF membranes and hence allow penetrationof CB into membrane pores. It can also damage PP at prolonged contacttime. Therefore, it is desirable for the solvent and contact time to becarefully chosen to avoid compromising the base membrane integrity.

EXAMPLE 2.3 Coating of Photo-thermal NMs via Covalent Bonding

Covalent bonding can be used to tether the photo-thermal NMs onto aporous membrane surface, if there are suitable functional groups presenton both the membrane and the photo-thermal NMs. Typical MD membranes donot have suitable functional groups on the surface. However,functionalization of —OH on PVDF, PTFE and PP membrane surfaces can beachieved through plasma treatment. Wet chemical oxidation, in KOH andKMnO₄ solutions for HF-elimination followed by nucleophilic addition inH₂SO₄ and NaHSO₄ solutions, can also be applied on PVDF membranes tointroduce —OH functional groups. Alternatively, polydopamine coating canbe applied to introduce functional groups on MD membranes as well.Photo-thermal NMs candidates include, without limitation, carboxylatedCB NPs and GO. Carboxylation of CB NPs can be achieved through oxidationin nitric acid or hydrogen peroxide under heated conditions.

A crosslinker is preferred to link the functional groups between thephoto-thermal NMs and the porous membrane surface. Crosslinkers that canbe used include, without limitation, trimesoyl chloride (TMC). A TMCsolution in isoparaffin-G can first be contacted with the functionalizedmembrane surface, during which time, TMC reacts with the —OH or —COOHgroups on the membrane. The TMC grafted membrane can then be soaked inGO or carboxylated CB NP suspension in THF to allow reaction between thefunctional groups on the NM and the —C═O on TMC (FIG. 3C). Specificallyfor the case of GO, due to its two-dimensional structure, multiplelayers of coatings can be achieved by alternatively applying thecrosslinker and GO suspensions onto the membrane surface to addadditional layers (FIG. 3D). Coating density can be varied by changingthe membrane modification conditions (e.g., plasma treatment time), CBconcentration in the suspension, and contact time with the CBsuspension.

EXAMPLE 2.4 Incorporating Photo-thermal Compositions in AdditionalPorous Polymer Layers

Photo-thermal NMs can be dispersed in a polymer-solvent mixture, andthen coated on the MD surface as an extra polymer composite layer viatechniques such as electrospinning. The type of polymers to chooseshould have minimal light absorbance. Candidates include, withoutlimitation, acrylonitrile, polymethyl methacrylate, polyethylene oxide,poly(vinyl alcohol), and the like. The photo-thermal NM-polymer-solventmixture is then applied to the membrane surface by electrospinning.Photo-thermal NM candidates include, without limitation, noble metalNPs, noble metal core-shelled structured NMs, and carbon based NMs, suchas CB NPs, and GOs.

Besides coating an additional composite layer with the photo-thermal NMsin the fibers of the polymer, the photo-thermal NMs can also be coatedon or embedded in a pre-synthesized macro-porous polymer film (orpolymer mesh), and then applied to the surface of the MD membrane.Polymers for the mesh preferably have minimal light absorbance.Potential candidates include, without limitation, acrylonitrile,polymethyl methacrylate, polyethylene oxide, poly(vinyl alcohol), andthe like. Similar coating methods, such as polydopamine coating andcovalent binding, can be used to coat the photo-thermal NMs on thechosen polymer mesh.

EXAMPLE 2.5 Use of Graphene Oxides as Photo-thermal Compositions

Graphene oxide (GO) nanosheets are quite hydrophilic and disperse wellin water. Therefore, they are preferably attached to base membranesurfaces via covalent bonds to prevent release of GO from the membranesurface. The same grafting methods used for CB NPs described above canbe used for GO, in which —COOH and —OH groups on GO react with TMCgrafted on base membrane surfaces (FIGS. 3C-3D). In this approach,Applicants only utilize the photo-thermal property of GO. Therefore, theorientation of the GO sheets, number of GO sheets in the coating layer,and the inter-sheet distance do not need to be controlled.

GO film's high water permeability is also an advantage. Typically, theMD membrane pore size is chosen by considering the trade-off betweenwater vapor permeability and membrane wetting. Membranes with smallerpores have higher liquid entry pressure, but lower water vaporpermeability. It has been shown that a membrane consisting of stacked GOsheets with inter-sheet distance between 6 and 10 Å allows water vaporto permeate through the nanocapillary network formed by the spacebetween GO sheets almost unimpeded in the form of a highly orderedmonolayer, where other vapors or gases cannot penetrate. This makes GOan optimal photo-thermal composition for MD membranes. GO has thepotential to provide high water permeability and very importantly,volatile organic contaminant removal, which cannot be achieved byexisting MD technology. However, fabrication of free-standing GOmembranes of large dimensions cannot be achieved with existingtechnology.

In this Example, Applicants can fabricate nanocomposite membranes byforming the GO film on a porous support membrane. This can allowmembranes with large pore sizes (e.g., a few micrometers) and evenhydrophilic materials to be used for MD. Because an intact, individualGO sheet is not permeable to water and the inter-sheet capillarypressure is extremely high (estimated to be up to 1000 bar), wetting isnot expected to be a concern. PVDF (Durapore SV), polycarbonate (IsoporeTT, isopore TS), and mix cellulose ester (type RA and SS) membranes withpore sizes from 1 to 5 μm (Millipore) can be tested as candidate supportmembranes.

GO coatings of a stacked structure can be obtained using three differentapproaches: spin coating, flow-directed assembly by filtration, andlayer-by-layer assembly with crosslinking. The first two methods havebeen used to prepare free-standing GO membrane or paper. However, it isexpected that the interactions between the hydrophilic GO andhydrophobic base MD membrane may be too weak to form strong attachment.To address this, the base membrane can be coated with polydopamine andreacted with TMC to provide covalent bonding between the membrane andthe GO coating. As the inter-sheet distance d is important to the waterpermeation rate, it can be tuned by heat annealing and chemicalreduction using hydrazine.

It has been reported that long-term immersion in water may lead toexpansion of inter-sheet space and potential salt penetration anddispersion of GO. Therefore, the long-term stability of the GO coatingcan be investigated at different temperatures and under various solutionconditions (i.e., pH, salinity, and divalent cations).

To enhance long-term GO coating stability, layer-by-layer assembly withcrosslinking can be used. In this method, TMC can be used to providebinding of the GO layer to the polydopamine coated support layer as wellas to cross-link GO sheets (FIGS. 3C-3D).

To properly align GO-sheets, each layer of GO can be deposited by spincoating followed by reaction with TMC. It is expected that theinter-sheet distance d can depend on the degree of cross-linking, whichcan be tuned by partial reduction of GO and adjusting the concentrationof TMC. This approach has been used to deposit up to 50 layers of GO ona polysulfone membrane. However, in previous studies, significantconvective flow of water through the GO-polysulfone membrane wasobserved at 50 psi. This suggests wetting of the GO coating, and iscontrary to the calculated extremely high capillary pressure and theobservation that pressure did not affect water permeation through a GOmembrane. Without being bound by theory, it is envisioned that suchdiscrepancy is attributed to the difference in the GO membrane/coatingthickness and structure resulting from the difference in the preparationmethods used, or the presence of coating defects.

GO membranes have also been prepared by spin coating, during which thestrong shear aligns the GO sheets to form a ordered, stacked structure.GO layers have also been assembled using a submersion protocol, duringwhich randomly oriented GO sheets were quickly cross-linked by TMC. TheGO coating was also much thinner (50 layers vs. 0.1-10 μm) and hencemore likely to have defects.

To properly align GO-sheets, each layer of GO can be deposited by spincoating followed by reaction with TMC. It is expected that theinter-sheet distance d can depend on the degree of cross-linking, whichcan be tuned by partial reduction of GO and adjusting the concentrationof TMC.

EXAMPLE 3 Assessment of Photo-thermal Membrane Performance

In this Example, Applicants assessed the performance of variousphoto-thermal membranes by utilizing the bench scale solar MD systemshown in FIG. 4. The bench scale system uses a direct contact membranedistillation (DCMD) configuration. The DCMD unit houses a flat sheetmembrane with an effective area of 28.27 cm² (8.10 cm×3.49 cm). The feedand permeate flow channels are both 0.3 cm in height. A quartz window of6.98 cm×3.49 cm allows irradiation of the membrane surface. Glass fibersheets with aluminum coating are applied to the surface of the DCMD unitexcept for the quartz window to minimize heating of the unit itself fromthe irradiation as well as heat loss to the environment. The whole unitis placed in a lab-made solar simulator, with an optical collimatorinstalled between the light source and the quartz window to minimizediffusive light. Simulated sunlight was provided by halogen tungstenlamps (FEIT MR16/GU10 120V 50W xenon) with the irradiation intensityadjusted by varying the number of lamps.

A peristaltic pump that circulates the feed and permeate flows throughthe DCMD unit, the temperatures of which are maintained by twoheating/chilling water baths with continuous monitoring of temperature.All tubings and connectors are wrapped with insulating materials tominimize heat exchange with the environment. An overflow design of thecold flow reservoir is used to collect the distillate, and a bench-topdigital balance monitors the accumulative mass of the distillate, fromwhich water vapor flux through the membrane can be determined. A flowthrough micro conductivity cell is installed on the concentrate permeatelines to continuously monitor permeate conductivity.

Liquid entry pressure (LEP) measurements were carried out on all of themembrane samples in this Example. LEP determines how much pressure isneeded to push water through the tested membrane. It is an importantparameter in the MD process to indicate the ability of the membrane toserve as the barrier between the feed and permeate liquid. LEPmeasurements were done by observing the protrusion of water through thetested membrane by gradually increasing the applied pressure.

In this Example, experiments were carried out using a commercial PVDFmembrane (0.2 82 m pore size), commercial nylon meshes (7 μm pore size),SiO₂/Au nanoshells (˜150 nm in diameter, 120 nm SiO₂ core), and CB NPs(Cabot, Inc., Billerica, Mass.). The base PVDF membrane and the NMmodified membranes were tested in a bench scale direct contact membranedistillation (DCMD) system with simulated solar irradiation (FIG. 4).The feed solution used was 1% NaCl, and the permeate was pure water. Thefeed and permeate temperatures were maintained at 30±0.1° C. and 20±0.1°C., respectively. Simulated sunlight was provided with six halogentungsten lamps (FEIT MR16/GU10 120V 50W xenon). Further details of themodification methods and results are summarized herein.

EXAMPLE 3.1 Coating of SiO₂/Au Nanoshells on Polydopamine Coated PVDFMembranes

For coating of SiO₂/Au nanoshells, PVDF base membrane was exposed to 2mg/ml dopamine chloride solution buffered at pH ˜8.5 with 10 mM Tris-HClfor 15 minutes. The membrane was dried in air and then exposed to anaqueous SiO₂/Au nanoshell suspension of ˜4.5×10⁹ particles/ml for 30minutes. The membrane was then dried in an oven at 60° C. for 2 hoursand thereafter rinsed thoroughly with pure water.

FIG. 9 summarizes the LEPs of various membranes before and aftermodification. The results indicate that, after the coating of SiO₂/Aunanoshells, the LEPs did not change significantly, and the membrane canstill effectively serve as the liquid barrier in MD applications.

FIG. 5A summarizes the performance of the base PVDF membrane, as well asthe SiO₂/Au nanoshells coated PVDF membranes under light irradiationconditions. FIG. 5A plots the increase in permeate mass in the unit ofmass per unit area of membrane (g/m²). The slopes of the curvesrepresent the permeate flux (g/m²-min). The SiO₂/Au nanoshell coatedmembrane has a flux of 60.11 g/m²-min, 20.7% higher than the basemembrane flux of 49.81 g/m²-min. Overall, the aforementioned resultsdemonstrate an increase in the slopes of the modified membranes whencompared with the base membrane.

EXAMPLE 3.2. Coating of CB NPs on PVDF Membrane

For coating of CB NPs, the membrane surface was exposed to 0.1 wt % CBNPs in chloroform for 1 minute. Then the suspension was removed and theresidual solvent on the membrane was allowed to dry. The obtainedmembrane was then thoroughly rinsed with pure water.

FIG. 9 summarizes the LEP before and after modification. The resultsindicate that, after the coating of CB NPs, the LEP did not changesignificantly and the membrane can still effectively serve as the liquidbarrier in MD applications.

FIG. 5B summarizes the performance of the PVDF membrane as well as theCB NP coated membrane. After coated by CB NPs, the membrane had a 33.5%increase in flux compared to the membrane with the unmodified PVDFmembrane (66.467 g/m²-min vs. 49.801 g/m²-min).

EXAMPLE 4 Electrospinning of CBNP-polymer Nanofibers

In this Example, Applicants demonstrate the fabrication of carbon blacknanoparticle (CBNP)-polymer nanofibers through electrospinning, and theutilization of the CBNP-polymer nanofibers as photo-thermalcompositions. This Example provides a new modification method for usingelectrospinning coating to form porous membranes. This is an example inparallel to the two examples included in Example 3.1 (AuNS coatedmembranes) and Example 3.2 (CBNP coated membranes).

EXAMPLE 4.1 Electrospinning of CBNP-polymer Nanofibers

An electrospinning scheme illustrated in FIG. 6 was utilized to createCBNP-polymer nanocomposite fibers with CBNPs trapped within the polymermatrix. Two types of polymers were used in the electrospinning process:poly(methyl methacrylate) (PMMA); and poly(vinyl alcohol) (PVA).

Electrospinning of the PMMA-CBNP composite was carried out using 20 wt %PMMA in Dimethylformamide (DMF) containing 0.5 wt % CBNP. The CBNP/PMMAmixture was mixed overnight and sonicated for 10 minutes beforeelectrospinning on the unmodified PVDF membrane.

The electrospinning was run at 15 kV applied voltage at an injectionrate of 1.5 mL/hour with the needle to a substrate distance of ˜15 cm. Atotal of 1 mL of injection volume was used for all the electrospinningexperiments. For abbreviation, “electrospun PMMA” (with no CBNP) and“electrospun PMMA-CBNP” fiber mesh coated membranes were designated as“M-ES” and “M-ESPC”, respectively.

Electrospinning of the PVA-CBNP composite was carried out using 13 wt %PVA in water. The PVA was modified with a styrylpyridinium group toallow for UV crosslinking of the fibers, providing stability in water.CB NP at 2 wt % concentration was dispersed in PVA solution usingsonication (35 W) for about 3 hours. Prior to electrospinning, the basePVDF membrane was coated with a thin layer of polydopamine (2 mg/mL, pH8.5, 15 minutes) to allow for strong adhesion between the hydrophobicPVDF and the hydrophilic PVA polymer fibers.

The electrospinning was run at 10 kV and an injection rate of 0.35 mL/h.The distance between the needle and the substrate was ˜10 cm.Electrospinning time was varied (5 minutes at 2 hr) to create fiber matsof different thickness. The membrane coated with electrospun PVAhydrogel fibers (with no CB NP) and electrospun PVA hydrogel fibers withCB NP are designated as “HM-ES” and “HM-ESPC”, respectively.

EXAMPLE 4.2 Characterization

As shown in the SEM images in FIGS. 7A-C, well-woven layers of thenanofiber mesh were formed on the membrane surface for both polymer-CBNPcombinations. The diameter of the fibers were fairly uniform.

The histogram of the PMMA-CBNP fiber diameters (FIG. 8A) showed a medianof 2.5 μm and an average of 2.47 μm. The thickness of the coated meshlayer was ˜100 μm, as characterized by the cross-section image in FIG.7B. Fiber diameter of the PVA-CBNP were comparatively much smaller,generally 400-450 nm (FIG. 8B).

Different concentrations of CB NP in the PVA solution were also testedand they did not seem to vary the fiber diameter significantly. Coatingthickness of PVA-CBNP layer increased with electrospinning time. Anelectrospinning of about 2 hours resulted in a coating thickness of 24μm (FIG. 11). Deionized water contact angle measurements showed thatmembranes modified with hydrophilic PVA fibers (FIG. 12B) were much morehydrophilic than membranes modified by hydrophobic PMMA fibers (FIG.12A). Moreover, Applicants observed that PVA fiber mats of sufficientthickness wet completely.

XPS analyses were carried out on the M-ESPC. The results are summarizedin Table 1. Theoretical carbon and oxygen concentrations and ratio inPMMA are included in Table 1 for comparison. Table 1 shows that carbonpercentage and C/O ratio in the fibers increased with the addition ofCBNP, suggesting successful incorporation.

TABLE 1 Elemental compositions of the PMMA and M-ESPC samples. Samples CO C/O PMMA (theoretical) 71.42% 28.57% 2.5 M-ESPC 75.49% 24.51% 3.1

FIG. 9 shows that the LEP of the membrane was the same before and afterthe coating of the electrospun mesh, suggesting that the membrane willfunction well as liquid barrier in MD.

The performance of the electrospun polymer-CBNP modified membranes weretested in the experimental setup shown in FIG. 4, with adjustments infeed temperature (now 35° C.) and feed and permeate channel height (now0.2 cm). Photo-thermal membranes described in Example 3.2 and 3.3 werealso tested in this new setup for comparison. Such results are alsosummarized in FIG. 10.

FIG. 10 summarizes the flux of the modified membrane. After coating ofthe photo-thermal mesh, the permeability of the membrane decreased, asshown by the decrease of flux of M-ESPC and HM-ESPC with no light.Without being bound by theory, it is envisioned that such results arelikely due to the increase of vapor transport resistance by theadditional hydrophobic photo-thermal mesh layer.

Without light, the M-CBNP samples and the M-AuNS samples had permeatefluxes comparable to those of the unmodified control, suggesting minimalalteration to membrane property. Reduced permeate fluxes were alsoobserved on both the M-ES and HM-ES samples. The M-ES had a 21% decreasecompared with the unmodified PVDF membrane. The HM-ES also showedsimilar decrease (17%) in flux. The effective coating created anadditional layer, which could increase the resistance of vapor transportand cause low permeability.

After turning on the light, increased permeate flux was observed on mostof the modified membrane samples. The M-CBNP had permeate flux of 102.8g/m²•min, a 14.52 g/m²•min increase from the unmodified base membrane.The M-AuNS had a permeate flux of 99.1 g/m²•min, which represented a10.6 g/m²•min increase from the base membrane.

Flux of the M-ESPC samples were comparable to the M-AuNS samples (99.3g/m²•min), likely due to the extra resistance form the mesh coating. Bycomparing the flux of the samples with and without light, the increasefrom the M-ESPC was the highest at 23.6 g/m²•min, suggesting that thecoated ESPC meshes were very effective in photo-thermal conversion.

The HM-ESPC had the highest flux performance of all samples, with 106.2g/m²•min and a 17.6 g/m²•min increase from the PVDF control. Thefindings show that embedding photo-thermal nanoparticles within ahydrophilic electrospun mat could decrease resistance to vapor andincrease contact between the photo-thermal nanomaterials (e.g., CB NP),thereby enhancing their performance.

Without further elaboration, it is believed that one skilled in the artcan, using the description herein, utilize the present disclosure to itsfullest extent. The embodiments described herein are to be construed asillustrative and not as constraining the remainder of the disclosure inany way whatsoever. While the embodiments have been shown and described,many variations and modifications thereof can be made by one skilled inthe art without departing from the spirit and teachings of theinvention. Accordingly, the scope of protection is not limited by thedescription set out above, but is only limited by the claims, includingall equivalents of the subject matter of the claims. The disclosures ofall patents, patent applications and publications cited herein arehereby incorporated herein by reference, to the extent that they provideprocedural or other details consistent with and supplementary to thoseset forth herein.

What is claimed is:
 1. A method of distilling a fluid, said methodcomprising: exposing the fluid to a porous membrane, wherein the porousmembrane comprises a surface capable of generating heat, and wherein theheat generated at the surface propagates the distilling of the fluid. 2.The method of claim 1, wherein the fluid is selected from the groupconsisting of water, alcohols, organic solvents, volatile solvents,water-alcohol mixtures, and combinations thereof.
 3. The method of claim1, wherein the distilling occurs by a membrane distillation method. 4.The method of claim 3, wherein the membrane distillation method isselected from the group consisting of direct-contact membranedistillation, air-gap membrane distillation, sweeping-gas membranedistillation, vacuum membrane distillation, and combinations thereof. 5.The method of claim 1, wherein the distilling results in fluiddesalination.
 6. The method of claim 1, wherein the distilling resultsin fluid purification.
 7. The method of claim 1, wherein the distillingresults in solvent separation.
 8. The method of claim 1, wherein theporous membrane is selected from the group consisting of polypropylene,polyvinylidene fluoride, polytetrafluoroethylene, polyethylene,polycarbonates, cellulose, and combinations thereof.
 9. The method ofclaim 1, wherein the porous membrane comprises a microporous membrane.10. The method of claim 1, wherein the porous membrane comprises poresizes ranging from about 0.2 μm to about 5.0 μm in diameter.
 11. Themethod of claim 1, wherein the surface spans an entire outer surface ofthe porous membrane.
 12. The method of claim 1, wherein the temperatureof the surface remains constant during distilling.
 13. The method ofclaim 1, wherein the surface is capable of generating heat when exposedto a light source.
 14. The method of claim 1, wherein the surface isassociated with a photo-thermal composition, wherein the photo-thermalcomposition generates the heat at the surface.
 15. The method of claim14, wherein the photo-thermal composition generates the heat at thesurface by converting light energy from a light source to thermalenergy.
 16. The method of claim 14, wherein the photo-thermalcomposition is selected from the group consisting of noble metals,semiconducting materials, dielectric materials, carbon-based materials,composite materials, nanocomposite materials, nanoparticles, hydrophilicmaterials, polymers, fibers, meshes, fiber meshes, hydrogels, hydrogelmeshes, nanomaterials, and combinations thereof.
 17. The method of claim14, wherein the photo-thermal composition is hydrophilic.
 18. The methodof claim 14, wherein the photo-thermal composition comprises a polymerselected from the group consisting of hydrophillic polymers, polymerfibers, electrospun polymers, functionalized polymers, and combinationsthereof.
 19. The method of claim 14, wherein the photo-thermalcomposition comprises carbon-based materials selected from the groupconsisting of carbon black, graphite, graphene, graphene oxide, reducedgraphene oxide, and combinations thereof.
 20. The method of claim 14,wherein the photo-thermal composition comprises nanoparticles.
 21. Themethod of claim 20, wherein the nanoparticles are selected from thegroup consisting of noble metal nanoparticles, metal oxidenanoparticles, semiconductor nanoparticles, gold nanoparticles,nanoshells, SiO₂/Au nanoshells, nanorods, carbon black nanoparticles,graphene nanoparticles, graphene oxide nanoparticles, reduced grapheneoxide nanoparticles, and combinations thereof.
 22. The method of claim20, wherein the nanoparticles are in the form an array on the surface.23. The method of claim 20, wherein the nanoparticles are embedded in ahydrophilic material coated on the surface.
 24. The method of claim 23,wherein the hydrophilic material is selected from the group consistingof polymers, meshes, fibers, mats, hydrogels, and combinations thereof.25. The method of claim 14, wherein the photo-thermal composition isdirectly associated with the surface of the porous membrane.
 26. Themethod of claim 14, wherein the photo-thermal composition is embedded ina polymer layer coated on the surface.
 27. The method of claim 26,wherein the polymer layer is selected from the group consisting ofpolystyrenes, polyacrylonitriles, polymethyl methacrylates,polydopamine, and combinations thereof.
 28. The method of claim 14,wherein the surface is associated with the photo-thermal compositionthrough at least one of covalent bonds, non-covalent bonds,physisorption, hydrogen bonds, van der Waals interactions, Londonforces, dipole-dipole interactions, and combinations thereof.
 29. Themethod of claim 14, wherein the photo-thermal composition iscross-linked to the surface.
 30. The method of claim 14, wherein thesurface is coated with the photo-thermal composition.
 31. The method ofclaim 30, wherein the photo-thermal composition is cross-linked withinthe coating.
 32. The method of claim 1, further comprising a step ofexposing the surface to a light source, wherein the light sourcefacilitates heat generation by the surface.
 33. The method of claim 32,wherein the light source is selected from the group consisting ofnatural light, ultraviolet light, visible light, near infrared light,laser, incandescent light, fluorescent light, LED light, light derivedfrom solar radiation, incident light, engineered light sources, andcombinations thereof.
 34. The method of claim 32, wherein lightintensity from the light source is amplified by a light amplifier. 35.The method of claim 32, wherein the light amplifier comprises opticallenses.
 36. The method of claim 1, wherein the heat generated at thesurface propagates the distilling by converting the fluid to a vaporthat flows through the porous membrane and condenses to a distillate.37. The method of claim 36, wherein the heat generated at the surfacepropagates the distilling by creating a temperature gradient across theporous membrane, wherein the temperature gradient results in formationof a vapor pressure gradient that drives the formed vapor through theporous membrane.
 38. The method of claim 36, further comprising a stepof collecting the distillate.
 39. The method of claim 1, wherein themethod occurs by only heating the fluid near the surface of the porousmembrane.
 40. The method of claim 1, wherein the method occurs withoutthe use of electric energy.
 41. A system for distilling a fluid, whereinthe system comprises a porous membrane, wherein the porous membranecomprises a surface capable of generating heat, and wherein the heatgenerated at the surface propagates the distilling of the fluid.
 42. Thesystem of claim 41, wherein the fluid is selected from the groupconsisting of water, alcohols, organic solvents, volatile solvents,water-alcohol mixtures, and combinations thereof.
 43. The system ofclaim 41, wherein the porous membrane is selected from the groupconsisting of polypropylene, polyvinylidene fluoride,polytetrafluoroethylene, polyethylene, polycarbonates, cellulose, andcombinations thereof.
 44. The system of claim 41, wherein the porousmembrane comprises a microporous membrane.
 45. The system of claim 41,wherein the porous membrane comprises pore sizes ranging from about 0.2μm to about 5.0 μm in diameter.
 46. The system of claim 41, wherein thesurface spans an entire outer surface of the porous membrane.
 47. Thesystem of claim 41, wherein the temperature of the surface remainsconstant during distilling
 48. The system of claim 41, wherein thesurface is capable of generating heat when exposed to a light source.49. The system of claim 41, wherein the surface is associated with aphoto-thermal composition, wherein the photo-thermal compositiongenerates the heat at the surface.
 50. The system of claim 49, whereinthe photo-thermal composition generates the heat at the surface byconverting light energy from a light source to thermal energy.
 51. Thesystem of claim 49, wherein the photo-thermal composition is selectedfrom the group consisting of noble metals, semiconducting materials,dielectric materials, carbon-based materials, composite materials,nanocomposite materials, nanoparticles, hydrophilic materials, polymers,fibers, meshes, fiber meshes, hydrogels, hydrogel meshes, nanomaterials,and combinations thereof.
 52. The system of claim 49, wherein thephoto-thermal composition is hydrophilic.
 53. The system of claim 49,wherein the photo-thermal composition comprises a polymer selected fromthe group consisting of hydrophillic polymers, polymer fibers,electrospun polymers, functionalized polymers, and combinations thereof.54. The system of claim 49, wherein the photo-thermal compositioncomprises carbon-based materials selected from the group consisting ofcarbon black, graphite, graphene, graphene oxide, reduced grapheneoxide, and combinations thereof.
 55. The system of claim 49, wherein thephoto-thermal composition comprises nanoparticles.
 56. The system ofclaim 55, wherein the nanoparticles are selected from the groupconsisting of noble metal nanoparticles, metal oxide nanoparticles,semiconductor nanoparticles, gold nanoparticles, nanoshells, SiO₂/Aunanoshells, nanorods, carbon black nanoparticles, graphenenanoparticles, graphene oxide nanoparticles, reduced graphene oxidenanoparticles, and combinations thereof.
 57. The system of claim 49,wherein the nanoparticles are in the form an array on the surface. 58.The system of claim 49, wherein the nanoparticles are embedded in ahydrophilic material coated on the surface.
 59. The system of claim 58,wherein the hydrophilic material is selected from the group consistingof polymers, meshes, fibers, mats, hydrogels, and combinations thereof.60. The system of claim 49, wherein the photo-thermal composition isdirectly associated with the surface of the porous membrane.
 61. Thesystem of claim 49, wherein the photo-thermal composition is embedded ina polymer layer coated on the surface.
 62. The system of claim 61,wherein the polymer layer is selected from the group consisting ofpolystyrenes, polyacrylonitriles, polymethyl methacrylates,polydopamine, and combinations thereof.
 63. The system of claim 49,wherein the surface is associated with the photo-thermal compositionthrough at least one of covalent bonds, non-covalent bonds,physisorption, hydrogen bonds, van der Waals interactions, Londonforces, dipole-dipole interactions, and combinations thereof.
 64. Thesystem of claim 49, wherein the photo-thermal composition iscross-linked to the surface.
 65. The system of claim 49, wherein thesurface is coated with the photo-thermal composition.
 66. The system ofclaim 65, wherein the photo-thermal composition is cross-linked withinthe coating.
 67. A method of making a membrane for distilling a fluid,wherein the method comprises: associating a surface of a porous membranewith a photo-thermal composition.
 68. The method of claim 67, whereinthe associating occurs by a method selected from coating, spraying,drop-casting, dip-coating, covalent binding, polymer blend coating,covalent crosslinking, spinning, electrospinning, and combinationsthereof.
 69. The method of claim 67, wherein the associating occurs byelectrospinning.
 70. The method of claim 67, wherein the porous membraneis selected from the group consisting of polypropylene, polyvinylidenefluoride, polytetrafluoroethylene, polyethylene, polycarbonates,cellulose, and combinations thereof.
 71. The method of claim 67, whereinthe photo-thermal composition is selected from the group consisting ofnoble metals, semiconducting materials, dielectric materials,carbon-based materials, composite materials, nanocomposite materials,nanoparticles, hydrophilic materials, polymers, fibers, meshes, fibermeshes, hydrogels, hydrogel meshes, nanomaterials, and combinationsthereof.
 72. The method of claim 67, wherein the photo-thermalcomposition is hydrophilic.
 73. The method of claim 67, wherein thephoto-thermal composition comprises a polymer selected from the groupconsisting of hydrophillic polymers, polymer fibers, electrospunpolymers, functionalized polymers, and combinations thereof.
 74. Themethod of claim 67, wherein the photo-thermal composition comprisescarbon-based materials selected from the group consisting of carbonblack, graphite, graphene, graphene oxide, reduced graphene oxide, andcombinations thereof.
 75. The method of claim 67, wherein thephoto-thermal composition comprises nanoparticles selected from thegroup consisting of noble metal nanoparticles, metal oxidenanoparticles, semiconductor nanoparticles, gold nanoparticles,nanoshells, SiO₂/Au nanoshells, nanorods, carbon black nanoparticles,graphene nanoparticles, graphene oxide nanoparticles, reduced grapheneoxide nanoparticles, and combinations thereof.
 76. The method of claim75, wherein the nanoparticles form an array on the surface.
 77. Themethod of claim 75, wherein the nanoparticles become embedded in ahydrophilic material coated on the surface.
 78. The method of claim 77,wherein the hydrophilic material is selected from the group consistingof polymers, meshes, fibers, mats, hydrogels, and combinations thereof.79. The method of claim 67, wherein the photo-thermal compositionbecomes directly associated with the surface of the porous membrane. 80.The method of claim 67, wherein the photo-thermal composition becomesembedded in a polymer layer coated on the surface.
 81. The method ofclaim 67, wherein the surface becomes associated with the photo-thermalcomposition through at least one of covalent bonds, non-covalent bonds,physisorption, hydrogen bonds, van der Waals interactions, Londonforces, dipole-dipole interactions, and combinations thereof.
 82. Themethod of claim 67, wherein the photo-thermal composition becomescross-linked to the surface.
 83. The method of claim 67, wherein thesurface becomes coated with the photo-thermal composition, and whereinthe photo-thermal composition becomes cross-linked within the coating.